Warming up is just as important as the workout itself. I play lacrosse and lift weights, therefore I warmup differently for each activity.
For lacrosse, I start with a light jog around the field. Then I do some active stretches, which are more calisthentics.
Then I finish with some sprints. For working out, I spend 10-15 minutes on the exercise bike or the treadmill.
Then I will grab just the bar and do either bench press, squat or deadlifts. Then I will do a quick stretch.